This invention relates to systems and methods for communication using optical networks, and more particularly to problems of minimizing polarization mode dispersion and chromatic dispersion in asymmetric grating assisted devices.
In optical networks, in which long distance optical fiber-based transmission systems are required to transmit signals at high (e.g. Gigabit) data rates, seemingly minor or minute physical or optical variants from the ideal can introduce anomalies which substantially degrade system capabilities. Most of these relate to dispersion, the tendency of optical waves or signals to separate or shift during propagation. One such anomaly pertains to an effect termed polarization mode dispersion (PMD). This effect derives ultimately from the fact that optical fibers, in practice, have birefringent properties and their refractive indices are polarization dependent. The birefringence in long length single mode fibers, for example, results from deviation of the fiber core from circular symmetry, from the presence of intrinsic and extrinsic stresses, and from curvature in the fiber along its length. In consequence of the polarization dependence, optical waves which are transmitted with different initial polarizations are subject to varying amounts of group propagation delays, the time differential between group delays increasing with propagation distance.
A different but generally related problem in optical systems is that of achieving polarization independence, so that a device which exhibits birefringence responds identically regardless of input signal polarization. In a previously filed application that is assigned to the assignee of the present invention, entitled xe2x80x9cMethods of Fabricating Grating Assisted Coupler Devicexe2x80x9d, by A. S. Kewitsch et al., Ser. No. 09/128/476, filed Aug. 4, 1998, now U.S. Pat. No. 6,169,830 B1, different techniques are disclosed for achieving polarization independence in a merged fiber, grating assisted coupler device such as an add/drop filter. One of these techniques employs twisting of the merged region about its longitudinal axis, which collapses the drop wavelengths of different polarizations to generate a common, polarization independent, drop passband. This is effective for a coupler that is useful as an add/drop filter and for other purposes. Twisting a coupler also can be useful to vary the power splitting ratio between outputs of a fused single mode fiber coupler, as described by T. A. Birks in an article entitled xe2x80x9cTwist-Induced Tuning in Tapered Fiber Couplersxe2x80x9d, in Applied Optics, Vol. 24, No. 19, 1 Oct. 1989, pp. 4226-4233.
The device described by Kewitsch et al. is non-evanescent and operates in a reflection mode to provide wavelength selectivity with high efficiency (the Birks coupler is evanescent and splits power without a recorded grating). Because of its low loss operation capabilities for add/drop operations at selected wavelengths, this approach satisfies basic requirements for many advanced optical fiber based systems. However, new performance targets are constantly being set, one of these arising from a demand for ever increasing data rates. Most current work is directed toward achieving data rates at 10 to 50 Gbs. While higher data rates are achievable because of the short wavelength of light, the effects of PMD are increasingly more severe, since differential group delays progressively degrade the closely spaced signal transitions at higher frequencies.
In grating assisted couplers, including the type mentioned above, the presence of a grating inherently introduces a non-linear dependence of phase response on wavelength. This non-linear phase response, together with non-circularity in the cross-section of the merged fibers in the region in which the grating is written, may raise the effect of PMD to several orders of magnitude higher than that inherent in the fiber itself. There are cross-sectional shapes (such as an ellipse with particular ellipticity) which result in low PMD; however, the shape tolerances necessary to achieve this reduction are impractical. For example the two merged fibers typically define a peanut or dumbbell shape, with one side of the merged region being larger than the other, pursuant to the non-evanescent coupler concept. In practice, precisely monitoring the elliptical shape in a manufacturing environment is extremely difficult and a need exists for other remedies. An internal grating coextensive with both fibers reflects signals of a selected wavelength from one fiber back along the other. In addition, as disclosed by Kewitsch et al., elliptical fiber geometries are formed during the initial drawing, such that LP01 and LP11 modes exhibit nearly equal and opposite birefringence. This corresponds to the general condition needed for polarization independence in the drop signal. However, taken together with the dispersion produced by the grating, the birefringence introduces high PMD. It is increasingly desirable further to minimize the PMD in the reflected (drop) channel, but to do so without diminishing the other advantageous properties of the coupler. A dispersion of less than about 1 ps over a 50 nm wavelength is a desirable objective.
In photonic add/drop couplers such as those discussed, chromatic dispersion (CD) also is a potential problem to be reduced to levels at which WDM signals at high data rates will not be degraded. The chromatic dispersion response is preferably to be held within 100 ps/nm in peak to peak amplitude over the add/drop passband, a property which has heretofore not consistently been achieved. Minimization of CD also contributes to minimization of PMD.
A grating assisted reflection mode coupler in accordance with the invention is non-evanescent and asymmetric, and establishes reflected light modes which are orthogonally polarized and have different wavelength shifted reflection amplitude and phase responses. In accordance with the invention, the various advantageous interactions and relationships of the coupler are not altered, but a number of controls are utilized during and after the formation of the grating-confining section and in the writing of the grating itself to minimize PMD and CD as well.
In the process of forming the fused fiber coupler waist by tension and a lengthwise reciprocating heat source, such as a torch, the states of polarization of light at different times through the fibers are monitored directly. At least three different states of polarization in the waist are sensed for each of the two lowest order modes, and from these readings the angles of rotation of the two lowest order modes on the Poincare sphere are determined. The heat and tension parameters are controlled to maintain a final condition in which the angles are equal and of opposite sign. When this occurs, the heat is reduced to a level at which the viscosity is sufficiently high for the shape of the waist to be preserved during subsequent elongation. At this point, the coupler not only has achieved a polarization independent state but one in which PMD has been minimized. To further minimize PMD while providing low CD, a saturated grating with substantially flat add/drop characteristics is written, such that the CD response within the add/drop passband varies less than 100 ps/nm in peak to peak amplitude. The additive benefits of angular displacement (twist) about the length of the waist region also can be utilized, to provide a PMD of equal to or less than 1 ps in add/drop response over a 50 nm wavelength span.